1. Field of the Invention
This invention relates to industrial physics and, in particular, to laser technology and machinery and, more particularly, to a method of laser treatment of an object and a device realizing this method.
2. Description of the Related Art
At present laser beams are used in many fields of science and technology, in industry and medicine to irradiate a specific zone on an object by a specific distribution of density of the laser flux in order to achieve an energy input for some sort of treatment. This problem is more or less effectively dealt with by several known methods of laser treatment using many different optical laser systems. But it should be admitted that potentialities of laser beams in treatment of various materials are far from being fully exploited. Since lasers are not cheap to use, each method and device is to be assessed on the basis of efficiency achieved in the application of lasers which are the basic component of such methods and devices.
Laser technologies can be classified according to the shape of the treatment zone into three types:
spot laser treatment--piercing blind and through holes, for example, spot welding; PA1 straight line laser treatment--welding, cutting, scribing; PA1 laser treatment of intricately-shaped patterns--heat treatment, lithography or printing, patterning.
The last and most complicated method of treatment of patterns consists in that, first, the optical system produces a laser beam in accordance with the required shape and size of the treatment zone (optical image production stage), and, second, a specific energy input is applied to this treatment zone (transmission of optical image to the object). The shape and size of the treatment zone are dictated by specific requirements of a production operation.
Several methods are used to produce an optical image for laser treatment of various objects. More popular are a contour-beam method and a mask method including contact and projection varieties.
Known in the art is a contour-beam method of laser treatment of an object (V. P. Veiko et al. Lazernaya Obrabotka, 1973, Lenizdat Publ., Leningrad, p. 144), wherein an optical image of a pattern is produced within a specific exposure period by successively illuminating a specific profile or contour by a light beam focused by an objective lens. The assigned contour is traced either by moving the object or by scanning the light beam. The object is usually placed in the focal plane of the objective lens and scanning is performed by mechanical transportation of the optical system of the device. It is important in this method that the optical system be equipped with at least one lens focusing radiation to a point and with a means for relative movement of the laser beam and the object.
This prior art method has the advantage of efficient use of laser energy. It is also good because it can achieve a high density of the energy flux in a spot area. But this method is deficient in that it is much less efficient in producing intricately shaped patterns because the whole treatment zone cannot be exposed at once. Besides, this method provides no optical means for producing a specific distribution of laser beam density in the treatment zone. These drawbacks restrict the range of production operations realizable by this method. If the treatment zone is not an outline or contour but a limited surface, this method cannot provide high quality of laser treatment.
Known in the art is a mask laser treatment method (V. P. Veiko et al., Lazernaya Obrabotka, 1973, Lenizdat Publ., Leningrad, p. 136), in which a laser beam of a specific shape and size is produced by irradiating areas on the surface of an object through masks (stencils) installed in the optical system of the device realizing this method. Free spaces or slots in the mask correspond to the required shape of the treatment zone, while the size can be left unaltered, reduced or magnified to a required size by the optical system. Two varieties of this method, contact and projection methods, have become popular recently.
The contact method consists in that a mask is pressed against the object prior to exposure. The advantage of this method consists in that the optical system of the device realizing this method is uncomplicated. But this method of laser treatment is deficient in that the object may be mechanically damaged when the mask is pressed thereto. On the other hand, if the mask is not intimately mated with the surface of the object, the quality of laser treatment is affected by diffraction distortions. Besides, the mask should be wear-resistant and immune to laser emission as compared to the material to be treated.
In the projection method the mask illuminated by the laser beam features slots corresponding to the desired shape of the treatment zone. The projection lens is used to demagnify the mask image to a desired size. The focal plane of the lens is matched with the surface of the object to be treated. There are several optical configurations employed in the projection method and, respectively, several different devices realizing this method (Lazernaya i Elektronno-Luchevaya Obrabotka Materialov, Reference book, N. N. Rykalin et al. 1983, pp. 445-449). This reference book also cites parameters of lasers to be used for laser treatment and industrial laser installations.
Mask laser treatment methods, both contact and projection ones, are deficient in that a large amount of laser energy is lost on non-transparent portions of the mask. Such methods cannot provide a desired distribution of laser radiation density over the entire zone of treatment.
Commonly known are combination methods where projection and contour techniques are used at the same time. In this case, the projecting optical system produces a reduced mask image in the focal plane of the objective lens, while scanners produce an optical image of the desired pattern in the conventional successive manner.
Known in the art is a device realizing a projection method of laser treatment of objects or materials (Elektronnaya Promyshlennost, Issue 1, 1976, Moscow, V. Z. Vysotsky et al., Ustanovka s Proektsionnoi Opticheskoi Sistemoi dlia Podgonki Resistorov, pp. 22-23) and intended for adjustment of parameters of passive components of integrated circuits. This prior art device comprises a laser radiation source and an optical system for delivery of laser radiation to the object to be treated, which is arranged on the optical axis of the laser radiation source. The manufacturing process of resistor adjustment consists in removing excess portions of resistive film stripes by evaporation of the material to a desired size by laser emission. For this purpose, the optical system of the device produces a laser beam spot in the desired plane as a 7-10 mm long and 1 mm wide straight line, the radiation density being distributed uniformly both in length and width. The device is equipped with a laser operating in a Q-switched mode at a wavelength of 1.06 micrometers. The output laser beam is focused by a positive lens to a cylindrical lens of the optical system, which is placed directly before the objective lens. A mask featuring a slot of a desired shape is placed behind the cylindrical lens. The generator of the cylindrical lens is oriented perpendicular to the slot so that the edges thereof are reproduced without distortion by a high-resolution objective lens. The plane of line images oriented across the slot is located outside the treatment zone and, consequently, the distribution of the laser beam density within the slot image is relatively uniform, slowly declining from the center towards the edges. The cutting length is restricted by the area of uniform energy density distribution by means of a special-purpose diaphragm located near the surface being treated.
This device is another striking example of the type of problems encountered when conventional optical elements are used to produce a desired shape of the treatment zone (a narrow strip, in this case) with a specific distribution of laser emission density (uniform, in this case) and maximum utilization of the radiated power. The optical system is inevitably overcomplicated and requires precision adjustment. Radiated power is inevitably lost on the mask.
Each known method of laser treatment of objects and devices realizing these methods have their merits and deficiencies, and their own fields of application. But there is no doubt that no existing method of laser treatment of specific patterns on objects and no existing device realizing this method can provide a combination of two functional capabilities such as a specific distribution of density of radiated power and concentration of the laser beam power within a treatment zone having a specific shape.